Ion-induced Hydrogel Formation and Nematic Ordering of

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Ion-induced Hydrogel Formation and Nematic Ordering of Nanocrystalline Cellulose Suspensions Pascal Bertsch, Stéphane Isabettini, and Peter Fischer Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01119 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 14, 2017

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Ion-induced Hydrogel Formation and Nematic Ordering of Nanocrystalline Cellulose Suspensions Pascal Bertsch,∗ Stéphane Isabettini, and Peter Fischer∗ Institute of Food Nutrition and Health, ETH Zurich, 8092 Zurich, Switzerland E-mail: [email protected]; [email protected] Phone: +41 44 632 85 36 Abstract Nanocrystalline cellulose (NCC) is a promising material for formation of hydrogels and nematically ordered phases. While salt addition is known to facilitate hydrogel formation, it remains unclear whether this originates from cationic bridging or charge screening effects. Herein, we demonstrate the effect of mono- and divalent salts on NCC gelation and nematic ordering. A strong correlation of NCC suspension zeta-potential and rheological behavior was found. Lower concentrations of divalent cations were needed to decrease NCC zeta-potential and form hydrogels. The same zeta-potentials and gel strengths were achieved at higher concentrations of monovalent salts. Saltinduced NCC aggregation is thus caused by intermolecular attractive forces rather than cationic bridging. Against excluded volume argumentation, salt addition was found to promote NCC nematic phase formation. Increased nematic ordering was observed in a transition regime of moderate salt addition before complete aggregation occurs. This regime is governed by an equilibrium of repulsive and attractive forces. Small angle neutron scattering suggests lateral orientation of NCC. Hence, NCC gelation and nematic ordering can be modulated via its zeta-potential by targeted salt addition.

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pletion, and hydrophobic attraction might occur. 6,22–24 Further, divalent cations may induce attractive forces between like-charged polyelectrolytes. 25,26 It remains unclear whether saltinduced NCC gelation is governed by ionic bridging or increased intermolecular attraction. Onsager’s original hard-rod model predicts lower nematic ordering capacity upon salt addition because of a decrease in excluded volume. Salt-induced NCC hydrogels are therefore expected to lose nematic properties, as supported by most experimental work. 5–8 However, our findings reveal the opposite phenomenon, stressing the need of revising the models of NCC nematic ordering behavior. We investigated NCC suspension properties and aggregation over a wide range of salt concentrations. Two mono- and divalent cations (Na+ , K+ , Mg2+ , Ca2+ ) were added as chloride salts to review potential cation specificity of NCC hydrogels. Nematic ordering of NCC was observed upon salt addition in a transition regime before complete aggregation occurs. This has been postulated for other polyelectrolytes that exhibit intermolecular attractive forces, but has not been reported for NCC. This allows to exploit NCC nematic ordering capacity below its established nematic transition concentrations.

Materials and Methods Materials. NaCl was purchased from Thermo Fisher (Zug, Switzerland) and CaCl2 (anhydrous) from Amresco (Solon, USA). KCl and MgCl2 (anhydrous) were purchased from Sigma-Aldrich (Buchs, Switzerland). Milli-Q water was obtained from a Merck Millipore system (Darmstadt, Germany) and D2 O (99.9 atom% D) from ARMAR Chemicals (Döttingen, Switzerland). NCC was kindly provided by CelluForce (Montreal, Canada). It consists of sulfated cellulose nanocrystals 80-150 nm in length and 2-10 nm in width and height, resulting in an aspect ratio L/D ≈ 20. 27 The electric double layer results in a higher excluded volume. Its radius can be approximated by the Debye length, which in water at room temperature is κ−1 (nm)

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√ ≈ 0.3/ I, where I is the ionic strength in mol/l. This is accredited by the effective diameter Def f . The native ion content of NCC was determined by Atomic Absorption Spectroscopy (AAS). 250 mg NCC was decomposed for 60 min with 4 mL 65% nitric acid (distilled inhouse) at 250 ◦ C and 40 bar. Atomic absorption was measured in triplicates with a Varian AA240 FS (Palo Alto, USA). The cation concentration per 1 g NCC was 6.500 ± 0.003 mg Na+ , 0.041 ± 0.057 mg K+ , 0.023 ± 0.022 mg Mg2+ , and 0.153 ± 0.001 mg Ca2+ . No significant effect on zeta potential or aggregation is expected at this cation concentrations. 21,28 Hence, the NCC powder was used without further dialysis. Salt concentrations indicated in this work represent the amount added, and do not include the native ion content. To assess the occurrence of counterion condensation the charge density of NCC was determined titrimetrically according to the SCAN-CM 65:02 protocol. Counterion condensation plays a role if lB ρ > 1, where lB is the Bjerrum length and ρ the linear charge density. The gravimetrically normalized charge density was 0.33 mmol/g. Assuming all charged groups on the surface of NCC this yields ρ = 0.6 nm−1 and lB ρ ≈ 0.4, indicating that counterion condensation does not occur. Methods. NCC Dispersion and Gel Preparation. NCC was suspended in Milli-Q under constant stirring. 1-5 wt% suspensions were produced. The resulting pH was 6-6.5 depending on NCC concentration. The chloride salts were added as a powder or in form of a 1 M salt solution for concentrations below 10 mM. 25 mL of NCC suspension were added under constant stirring until the salt was dissolved. Zeta-Potential Measurement. Zeta-potential was determined with a Malvern Zetasizer nano ZS (Malvern, UK) at 20 ◦ C. A 0.1 wt% NCC suspension was produced and different salt concentrations added as described above. At 0.1 wt% NCC suspension pH was 7. The zeta-potential of NCC suspensions is independent of pH in this range (pH 2-10). 28 Rheological Analysis. Rheological experiments were performed with a Physica MCR 501 shear rheometer (Anton Paar, Graz, Austria) equipped with a CC27 Couette geometry. Samples were oscillated in time sweep experiments at constant frequency ω = 1 rad/s until

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dynamic moduli reached constant values. The strain amplitude was constant at 0.5 or 1% based on results of previous amplitude sweeps. Frequency sweeps were conducted from 0.1100 rad/s. Steady shear experiments were performed from 0.1-100 1/s to verify applicability of the Cox-Merz rule. 29 Samples were covered with a solvent trap and temperature controlled to 20 ◦ C with a Peltier element. Polarized Light Photographs. Samples were filled into 25 mL Wheaton glass flasks L6225 (Wheaton, USA) and kept at room temperature for 10 days. The flasks were turned upsidedown and left for 10 days before taking pictures. Samples were placed between cross-polarizer with a light source behind it and pictures taken with a Nikon D800E equipped with a Nikon 55 mm macro lens (Tokyo, Japan). Small Angle Neutron Scattering (SANS). SANS measurements were conducted at the SANS-I beamline at PSI, Villigen, Switzerland. Sample preparation in D2 O was performed as introduced above. The sample was loaded in a 2 mm thick quartz cuvette (Hellma, Germany) and aligned in the neutron beam. The empty cuvette and Milli-Q water were recorded for background corrections. The neutron wavelength was fixed at 0.8 nm and a 2D 3

He detector was placed at 2, 6, and 18 m from the sample to cover a q-range from 0.05 to 1.5

nm−1 . Radially averaged scattering curves were computed from the 2D neutron scattering patterns of the sample using the BerSANS software.

Results and Discussion Zeta Potential Measurements. The zeta-potential characterizes the electrostatic interactions of particles in colloidal dispersions. The surface charge of NCC derives from its sulfate ester groups. Consequently, NCC has a negative zeta-potential over a wide pH-range, allowing the formation of charge stabilized suspensions. NCC zeta-potential can be altered by chemical modifications or increasing ionic strength. 18,30 In Figure 2 the influence of different chloride salts on NCC suspension zeta-potential is depicted.

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at lower salt concentrations. The maximum values of G’ were the same for all 4 salts (e.g. ≈ 2.5 kPa at 5 wt% NCC). This suggests that NCC aggregation and hydrogel formation is independent of cation valency. Instead it is determined by the decrease in electrostatic repulsion. Below a critical zeta-potential, NCC intermolecular attractive forces dominate inducing aggregation. This effect is observed at lower divalent cation concentrations. Divalent cations are known to induce attractive forces within like-charged polyelectrolytes. 25,26 This effect could further enhance NCC aggregation compared to monovalent cations. Our values for G’ are in good agreement with salt-induced NCC gels measured by Lenfant et al. 20 Chau et al. 21 reported values about one order of magnitude higher. Both groups proposed potentially stronger gels formed with divalent cations by intermolecular bridging. Our present data does not support this theory. Maximum gel strength was found to be independent of cation valency and radius. Although, divalent chloride salts induced gel formation at lower concentrations by more efficient charge screening. Dong et al. 32 found higher gel strength with increasing cation valency for cellulose nanofibrils, which are longer and more flexible compared to NCC. It was primarily attributed to intramolecular cationic cross-linking inducing a more compact structure with efficient charge screening. We assume this discrepancy between cellulose fibrils and NCC derives from the inability of short NCC rods to bend and form intramolecular cationic bridges. The aggregation of stiff NCC rods thus depends on sufficient charge screening until attractive forces dominate. Concerning the shear viscosity, the addition of salt has a shear rate dependent effect, as illustrated exemplarily for 3 wt% NCC in Figure 5. All samples displayed shear thinning behavior, originating from the orientation of rod-like NCC crystallites under shear. Without salt addition the viscosity curve leveled at low shear rates approximating a zero shear viscosity η0 . The addition of small amounts of chloride salts resulted in decreased viscosity. This derives from the electroviscous effect as discussed previously. For higher salt concentrations the slopes remained constant at decreasing shear rates denoting a yield stress τ0 . This suggests structure build based on increased NCC intermolecular interactions.

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breakup of a previous structure under shear, indicating the presence of nematic domains. This correlates with the onset of nematic ordering determined visually in Figure 6. Addition of 10 mM NaCl (Figure 7B) resulted in good agreement of the Cox-Merz rule for 1 and 2 wt% NCC, despite presence of nematic ordering (see Figure 6). The time frame of the experiment probably was insufficient to capture nematic ordering at these concentrations. At 3 wt% and higher NCC concentrations, deviating η and η* were observed owing to nematic domains. This effect was more pronounced when 2.5 mM CaCl2 were added (Figure 7C). Viscosity mismatch and failure of the Cox-Merz rule occurred at all NCC concentrations tested. This discrepancy to NaCl could derive from a lower zeta-potential allowing (see Figure 2) or attractive forces induced by divalent cations. 25 Similar Cox-Merz rule failure was previously reported above critical NCC nematic transition concentrations. 14,15 The addition of salt could allow to reduce NCC concentrations necessary for nematic ordering. Onsager’s original theory is based on a hard-rod repulsive potential, and is complicated by the presence of electrostatic interactions. Stroobants et al. 10 expanded Onsager’s approach by the twisting effect deriving from the angular dependence of electrostatic interactions. Consideration of the twisting effect does predict a decrease in nematic phase transition concentration for short and highly charged polyelectrolytes. Onsager’s prediction was seen to fail in such cases as it omits electrostatic interactions on higher virial terms. 33,34 It appears that intermolecular attractive forces further complicate the case. Most previous work reports decreasing NCC nematic phase volume with increasing ionic strength, in line with excluded volume argumentation. 5–8 However, salt concentrations were usually lower than those employed here (< 5 mM of monovalent salts). Stabilizing effects of salt on nematic phases were previously reported for various peptides. 24,35,36 Pelletier et al. 37 found that salt stabilizes nematic phases in V2 O5 gels. In all cases it was attributed to increasing intermolecular interactions, most commonly van-der-Waals forces. This could also apply for NCC. Although, depletion forces might play a role and NCC is known to interact hydrophobically. 6,22,23 This supports our finding of a salt-induced nematic transition regime based on increased inter-

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orients laterally at increasing NCC concentration driven by a space filling mechanism, what was considered the onset of liquid crystal formation. We assume that the charge screening of salt promotes this orientation at lower concentration. Therefore, the targeted salt addition allows to exploit NCC nematic ordering capacity below its established nematic transition concentrations. This might be applicable for other materials capable of forming nematic phases. Peptides are of particular interest due to the relevance of protein assembly in DNA and physiological processes. Several studies already delivered the proof of concept for F-actin and microtubules. 24,35,36

Conclusions The effect of mono- and divalent salts on NCC suspension gelation and nematic ordering capacity was investigated. We found a strong correlation of NCC suspension zeta-potential and storage modulus G’. Lower divalent cation concentrations were needed to decrease zetapotential and induce hydrogel formation due to more efficient charge screening. However, equivalent zeta-potentials and gel strengths were reached at higher monovalent cation concentrations. This suggests that NCC aggregation is independent of cation valency, but based on charge screening allowing intermolecular attraction. Consequently, NCC hydrogel properties can be modulated via its zeta-potential by targeted salt addition. We postulate a transition regime at moderate salt concentrations where NCC forms nematically ordered phases, as observed optically and characterized rheologically. This transition regime is governed by an equilibrium of repulsive and attractive forces within NCC crystallites. We propose lateral aggregation of NCC as supported by SANS. This nematic ordering mechanism could be of general interest for polyelectrolytes exhibiting attractive forces, as it paves the way to exploit nematic orientation properties below established nematic transition concentrations.

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Author Information Corresponding authors Pascal Bertsch, [email protected] Peter Fischer, [email protected] Notes The authors declare no competing financial interests.

Acknowledgement The authors thank CelluForce for providing nanocrystalline cellulose and Christophe Zeder for performing AAS. Joachim Kohlbrecher for assisting in SANS experiments at the Swiss Spallation Neutron Source (SINQ) at PSI, Villigen, Switzerland. Further Michael Diener and Antoni Sánchez-Ferrer for their valuable inputs and discussions. This project was funded by the Swiss National Foundation, Project No. 2000-21137941.

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